Susan D. Benecchi

The NSF-DOE Vera C. Rubin Observatory’s combination of survey speed, sky coverage, and depth provides a unique opportunity to study the distant trans-Neptunian region, including the Neptune Trojans. The Neptune Trojans are small bodies on orbits with Neptune’s semimajor axis that are locked in the 1:1 mean motion resonance librating about the ice giant’s stable Lagrange points, L4 (leading) and L5 (trailing), located 60 ^∘ ahead of or behind the planet. For more than 15 yr, the on-sky location of the L5 Neptune Trojan cloud has been coincident with the Galactic plane and Galactic center; the high stellar density has significantly hindered the discovery of L5 Neptune Trojans. In 2025, the L5 cloud had moved sufficiently out of the Galactic plane to overlap with the sky covered by Rubin Observatory’s Science Validation (SV) survey that was executed during the observatory’s commissioning phase. We report on two new confirmed long-term stable (primordial) members of the Neptune L5 Trojan cloud (2025 MH348 and 2025 NN80), doubling the known sample, and one candidate (2025 MD138), all of which have been found in a preliminary search of the SV survey observations. We present our study to characterize the physical and dynamical properties of these objects and compare the L5 sample to the well-studied L4 cloud. This work also presents the first characterization of objects discovered by Rubin Observatory’s moving object discovery pipelines using observations taken in an LSST-like cadence, and as such, provides a blueprint for LSST-era analyses.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object’s region of the sky during routine commissioning. Facilitated by Rubin’s high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of η ≳ 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g − r = (0.657 ± 0.013) mag, r − i = (0.235 ± 0.018) mag, i − z = (0.147 ± 0.042) mag, and z − y = (0.047 ± 0.052) mag. These data represent the earliest observations of this object by a large (≳8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time will provide after it begins in early 2026.

The Colours of the Outer Solar System Origins Survey (Col-OSSOS) measured the optical/near-IR colors of a brightness-complete sample of trans-Neptunian objects (TNOs). Like previous surveys, this one found a bimodal color distribution in TNOs, categorized as red and very red. Additionally, this survey proposed an alternative surface classification scheme: FaintIR and BrightIR. Cold classical TNOs mostly have very red or FaintIR surfaces, while dynamically excited TNOs show a mixture of surfaces. This likely indicates that formation locations and proximity to the Sun influenced surface characteristics and color changes. Our study combines the data from Col-OSSOS with two dynamical models describing the formation of the Kuiper Belt during Neptune’s migration. We investigate the proposed surface-color-changing line and explore the distribution of different surfaces within the primordial disk. By comparing radial color transitions across various scenarios, we explore the origins of surface characteristics and their implications within the context of BrightIR and FaintIR classifications. Moreover, we extend our analysis to examine the distribution of these surface classes within the present-day Kuiper Belt, providing insights into the configuration of the early solar system’s planetesimal disk prior to giant planet migration. We find that the most likely primordial disk compositions are inner neutral/outer red (with transition 30 . 0 − 1.2 + 1.1 au) and inner BrightIR/outer FaintIR (with transition 31 . 5 − 1.2 + 1.1 au).

We report on roughly 16 yr of photometric monitoring of the trans-Neptunian binary system (120347) Salacia–Actaea, which provides significant evidence that Salacia and Actaea are tidally locked to the mutual orbital period in a fully synchronous configuration. The orbit of Actaea is updated, followed by a Lomb–Scargle periodogram analysis of the ground-based photometry, which reveals a synodic period similar to the orbital period and a peak-to-peak lightcurve amplitude of Δ m = 0.0900 ± 0.0036 mag (1 σ uncertainty). Incorporating archival Hubble Space Telescope photometry that resolves each component, we argue that the periodicity in the unresolved data is driven by a longitudinally varying surface morphology on Salacia, and we derive a sidereal rotation period that is within 1 σ of the mutual orbital period. A rudimentary tidal evolution model is invoked that suggests synchronization occurred within 1.1 Gyr after Actaea was captured/formed.

Trans-Neptunian objects (TNOs), icy remnants of planet formation orbiting beyond Neptune, comprise a dynamically and compositionally diverse population of >5000 known members. An extensive body of literature focuses on the dynamical structure of the trans-Neptunian region. Less information is available on TNO compositions due to the difficulty carrying out spectral observations of objects dozens of astronomical unit from the Sun. The James Webb Space Telescope has revolutionized our understanding of TNO surface compositions by enabling near-infrared spectroscopy out to ∼5 μm. Clustering and principal component analysis reveal that the majority of intermediate-size TNOs fall into one of three broad spectral classes. Here we propose a more descriptive taxonomy based on the most spectrally prominent features for each class: H₂O-, CO₂-, and organics-types. Additional categories may arise via future JWST observations; the naming convention described here is extensible to any newly identified (sub-)categories based on prominent spectral features.

We report on roughly 16 years of photometric monitoring of the transneptunian binary system (120347) Salacia-Actaea which provides significant evidence that Salacia and Actaea are tidally locked to the mutual orbital period in a fully synchronous configuration. The orbit of Actaea is updated, followed by a Lomb-Scargle periodogram analysis of the ground-based photometry which reveals a synodic period similar to the orbital period and a peak-to-peak lightcurve amplitude of δm = 0.0900 {\pm} 0.0036 mag (1σ uncertainty). Incorporating archival HST photometry that resolves each component, we argue that the periodicity in the unresolved data is driven by a longitudinally varying surface morphology on Salacia, and derive a sidereal rotation period that is within 1σ of the mutual orbital period. A rudimentary tidal evolution model is invoked that suggests synchronization occurred within 1.1 Gyr after Actaea was captured/formed.

We propose a Roman Space Telescope survey to investigate fundamental properties of the distant solar system in the region of the Kuiper Belt where object characteristics and the size distribution are inaccessible from any other telescope. Our pointing is coincident with the search space accessible to NASA's New Horizons spacecraft meaning, that a discovered object sufficiently near the orbit of New Horizons would potentially be investigated by a close flyby. In addition, numerous objects expected to be discovered by this search can be observed in the distance by New Horizons allowing their surface properties and satellite systems to both be probed. As designed, this survey will discover and determine orbits for as many as 900 Kuiper Belt objects (KBOs), providing a unique opportunity for ground-breaking Kuiper Belt science. It will simultaneously: (1) Probe and characterize the deep Kuiper Belt by identifying objects as small as a few km and taking our understanding of the size distribution to a new level. This has implications for understanding the the standard model (the Streaming Instability) of KBO formation and elucidating crater formation physics on these icy bodies. (2) Open KBO rotation studies, in particular of those objects with long rotation periods,(3) Discover and characterize KBO binaries at large distances, important because their duplicity offers information about object densities at these distant locations from the Sun. (4) Shed light on the cratering history of KBOs and improving the dating of the surfaces of Arrokoth, Pluto and Charon in addition to helping to place the 32 distant KBOs New Horizons has observed in context. This project also has synergies with transiting exoplanet studies due to the stellar density of our search fields. Coupled with our timing requirements it is sensitive to discovery of hot Jupiters and hot Neptunes.